The August 27 incident, which Japan called regrettable, came amid widespread anti-Japan demonstrations in China over a disputed East China Sea island chain known as Diaoyu in China and Senkaku in Japan.

Ambassador Uichiro Niwa was not hurt in the incident and there was no other damage to his diplomatic vehicle, according to the Japanese embassy.

Beijing police detained a 23-year-old man surnamed Guo and a 25-year-old man surnamed Xia as the chief suspects in the incident, the city government's official news website Qianlong reported.

The two men, who both admitted the wrongdoing, were "administratively detained", the report said, which means they are likely to face lesser punishments than if they were placed in criminal detention.

Another man, who allegedly used his car to stop the ambassador's vehicle allowing the flag-snatch to take place, was issued a warning, it added.

China last week said it was "seriously investigating" the incident, state media said.